Build-up of internal cake in layered filtering media

R. Kirsch, S. Osterroth*, S. Rief, Fraunhofer Institut für Techno- und Wirtschaftsmathematik (ITWM), Germany

The properties of a filtering medium can be customized by considering a layered composition. Each layer can have different properties such as porosity, pore size distribution, or permeability. A major goal of such a composition is to distribute a polydisperse dust more evenly over the depth of the medium. Depending on the fractional efficiency properties, media with only one (homogenized) layer tend to be loaded mostly in the upstream region of the medium, whereas in many cases, the downstream region of the medium is poorly penetrated by the dust (or only reached or even passed by the smallest particles). Therefore, graded or layered media are frequently used to improve dust holding capacity, filtering efficiency, and pressure drop behavior of the filtering medium.

To this end, the upstream layers are chosen as relatively open in order to prefilter the large particles. Consequently, the downstream layers are more dense and provide the filtration efficiency for the small particles. The challenge for the media designers is to find an optimal combination of filtering materials together with a suitable thickness of the layers. Problem-adapted modeling and numerical simulation can accelerate this stage of the product development. In many cases, direct numerical simulations of the graded or multilayered media that resolve the dust particles and the pore spaces of the media in the computational grid are costly. Therefore, one is interested in using macroscopic models based on effective material properties that allow for much faster computations. However, this approach requires the knowledge of the proper values of the model parameters. This suggests the development of a simulation method combining the advantages of both approaches.

In general, a filter cake grows on the surface of a filtering medium as soon as a critical porosity in the first upstream layer is reached. This equivalent to the fact that the largest particles can not enter the medium any longer because the pores are too narrow. If for example, the medium is made of two layers and the downstream one has a lower porosity than the upstream, it might happen that the critical porosity is reached earlier in the downstream layer. This means that a cake starts to grow in the structure of the upstream layer. This phenomenon is often called internal cake filtration.

This study is devoted to macroscopic modeling of internal cake filtration in layered filtering media...

Session: L5 - Depth Filtration - Modelling and Design
Day: 14 March 2018
Time: 09:00 - 10:15 h

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